1. Field of the Invention
The present invention relates to a magnetic recording medium having a magnetic layer of a metal thin-film type and a process for producing same.
2. Description of the Related Art
As magnetic recording media for high-density magnetic recording, so-called metal thin-film type magnetic recording media have been proposed. In these so-called metal thin-film type magnetic recording media, a metallic magnetic material, such as a Co—Ni alloy, a Co—Cr alloy, Co—O and the like, is deposited on a non-magnetic support member, such as a polyester film, a polyamide film, a polyimide film and the like, by a plating process or a vacuum thin-film forming technique, including a vacuum deposition (evaporation) process, a sputtering process, an ion plating process and the like.
Because such metal thin-film type magnetic recording media have a number of advantageous characteristics, they are mainstream in the field of high-density magnetic recording. One of the advantages the metal thin-film type magnetic recording media have is that, due to the fact that they have superior coercive force and squareness ratio, and that the thickness of the magnetic layer can be made extremely thin, they have superior electromagnetic conversion characteristics at short wavelengths and demagnetization during recording and thickness loss during reproduction are extremely small. In addition, because, unlike in metal particle (MP) magnetic recording media, a binder, which is a non-magnetic material, is not incorporated into the magnetic layer in the metal thin-film type magnetic recording media, the packing density of the ferromagnetic metal particles can be increased.
Further, so-called oblique-angle deposition type magnetic recording media have been put to practical use. In the oblique-angle deposition type magnetic recording media, a magnetic layer is formed by depositing a metallic magnetic material at an oblique angle in order to improve electromagnetic conversion characteristics and obtain larger output.
In order to reduce spacing loss in the metal thin-film type magnetic recording media to accommodate high-density recording, there is a trend towards smoother surfaces for the magnetic layer. However, as the surface of the magnetic layer becomes smoother, the contact area between the magnetic layer and a magnetic head becomes larger, thereby increasing the frictional force therebetween as well as the shear stress experienced by the magnetic layer. In order to protect the magnetic layer under such harsh conditions, it is important to form a protective layer on the magnetic layer.
For the protective layer that protects the magnetic layer, for example, a carbon film, a quartz (SiO2) film, a zirconia (ZrO2) film and the like can be used, and these films have already found practical applications in hard disks. As a material for the protective layer, of carbon films, particularly hard carbon films having a diamond structure (so-called diamond-like carbon films) are effective and are currently widely used in practical applications.
Examples of processes for depositing the hard carbon film mentioned above include a sputtering process and a plasma chemical vapor deposition (hereinafter referred to simply as plasma CVD) process.
In the sputtering process, a sputter gas, such as argon (Ar) gas, is ionized (plasmarized) and accelerated using an electric field or a magnetic field, and the resultant ions are bombarded on a target surface. Target atoms are sputtered from the target when the plasma particles hit the target, and these sputtered atoms are deposited on a base material to form a sputter film.
However, when a hard carbon film is deposited by the sputtering process, the deposition rate is generally slow and hence therein lies a problem in that the process, from a commercial standpoint, is poor in terms of productivity.
On the other hand, in the plasma CVD process, the energy of the plasma generated in the electric field causes a reactant gas, which becomes a material for the film to be formed, to under go a chemical reaction, such as decomposition or synthesis, and the product obtained from the chemical reaction is deposited on a base material to form a CVD film.
The plasma CVD process has an advantage in that the deposition rate is higher as compared to the sputtering process, and therefore it is viewed favorably as a promising process for forming hard carbon films.